Topicity

The stereochemistry of the topicity denotes the stereochemical relationship of substituents relative to the structure with which they are associated. Depending on the relationship, such groups may homotopic, enantiotopic, or diastereotopic be. One speaks of Homotopizität, Enantiotopizität, or Diastereotopizität.

Homotopizität

Homotopic groups in chemical compounds are equivalent groups. It refers to groups or atoms as homotopic, if the molecule when replacing one of the groups remains achiral, or an existing chirality does not change. Typical examples are the two hydrogen atoms in dichloromethane or the hydrogen atoms in the methyl groups. Homotopic atoms or groups of the same chemical shifts in nuclear magnetic resonance spectroscopy (NMR).

Enantiotopizität

The stereochemical term enantiotopic refers to two groups, which, if they replaced, enantiomers would result.

For example, the two hydrogen atoms are enantiotopic, which are bonded to the second carbon atom of butane. If you replace one of the hydrogen atoms by bromine, the result is (R )-2 -bromobutane. If one replaces the other hydrogen atom by bromine, the result is the enantiomer, ie (S )-2 -bromobutane.

Also enantiotopic atoms or groups of the same chemical shifts in nuclear magnetic resonance spectroscopy. ( As long as measured in a non-chiral environment).

Diastereotopizität

The stereochemical term diastereotopic refers to the position of two groups when they are replaced ( substituted ), resulting diastereomers. For example, the two hydrogen atoms are diastereotopic at the third carbon atom of ( S)-2 -bromobutane. By replacing one of the hydrogen atoms (pictured in blue) by bromine, the result is (2S, 3R ) -2,3- Dibromobutane. If one replaces the other hydrogen atom (in the picture shown in red ) by bromine, the result is the diastereomer, ( 2S, 3S) -2,3- Dibromobutane.

Diastereotopic atoms or groups can show in nuclear magnetic resonance spectroscopy ( NMR) different shifts, depending on how strong is the influence of the chiral environment on the two atoms or groups. The decisive factor is the distance of the diastereotopic groups from the chiral center and the conformational flexibility of the molecule. Diastereotopic atoms in relatively conformationally molecules with several nearby stereocenters often show strikingly large differences in the chemical shift. Typical examples of this are the CH2 group in carbohydrates ( position 6 in hexoses ) and the side chain protons, especially the β - protons ( HB), the amino acids folded proteins.